Radio transmission systems, e.g. long-haul radio transmission systems, may suffer from channel fading phenomena. For example, weather-related reflections or refractions cause multipath reception with severe signal distortions and dramatic loss of receive power. Such fading phenomena may also be referred to as “deep fading”.
One known countermeasure against deep fading is diversity reception, in which receive signals of two antennas with different positions, i.e. signals from two receive branches, are combined. The careful choice of the antenna positioning may help to reduce deep fading on both receive branches at the same time.
Some diversity reception methods combine analogue signals from the two receive branches in an intermediate frequency range. Because only signals with equal transmission delays can be combined, delay compensation is accomplished before the signals are combined. The delay equalization may be implemented in the analogue domain using delay compensation cables with individually tailored lengths. However, in this case, an individual delay compensation cable needs to be configured and mounted for every radio station, which is time consuming and expensive.
Other diversity reception methods use digital combiners. In this case, fully-automatic delay equalization methods for digital signals are available. In a typical digital diversity combiner two adaptive filters are used, one in each of the receive branches. The adaptive filters are similar to an adaptive channel equalizer as used for non-diversity receivers. In the case of using digital combiners, the signal processing tasks of channel equalization and signal combination are joined together and can no longer be separated.
In a digitally implemented diversity receiver, a symbol timing recovery is needed for every analog-to-digital conversion. There are basically two alternatives. The first alternative is a “synchronous” sampling method, in which a physical clock is controlled with the aim to sample the input signals at the timing positions of the symbols. The second alternative is an “asynchronous” sampling method, in which the physical clock and the sampling of the input signal is free-running. In the latter case, the symbols may be recomputed from the input samples using interpolation filters.
In the first alternative, the physical clock may be controlled using a phase control loop in which a timing error detector is digital and an analog voltage controlled oscillator is used. Accordingly, in the first alternative, the phase control loop is partly digital and partly analog. In the second alternative, the symbol timing recovery may be fully digital and implemented in an ASIC (“Application Specific Integrated Circuit”).
Irrespective of the choice of synchronous or asynchronous sampling, because different fading may occur on the two receive branches of a diversity receiver, it is known to provide an individual timing recovery function for each of the two receive signals.
However, in such a diversity receiver with digital combination and an individual symbol timing recovery for each receive branch, there may still be problems due to special fading phenomena, which produce cycle slips in the recovered symbol timing recovery.
A cycle slip is a recovery error, which is caused by a temporary loss of synchronization between a recovered clock signal and an input signal, from which the clock signal is to be recovered, due to a phase error between the recovered clock signal and the input signal being in excess of a full cycle. In other words, a clock recovery method typically estimates the sampling time of a received signal only with an ambiguity of an integer multiple of a symbol duration. After a deep fading event, the recovered sampling time may have shifted by one symbol duration as compared to the sampling time before the deep fading event, which is then referred to as a “cycle slip”. In a digital diversity receiver, cycle slips adversely affect the common operation of the adaptive filters used for combining the receive signals.
Cycle slips can be seen as additionally introduced delays in the signal paths. There is a tendency of the cycle slips causing the adaptive filters to counteract these delays. This is undesirable, because due to their limited length the adaptive filters are only able to compensate a limited number of cycle slips. Moreover the performance of the adaptive filters degrades considerably after a cycle slip has occurred. Cycle slips may occur seldom, but every occurrence has significant consequences on the link transmission quality.
Accordingly, there is a need for improved techniques for symbol-timing recovery in a multi-branch receiver.